| The alteration of starch molecular structure can induce the corresponding changes in its physical and chemical properties.Therefore,exploring the fine structure of starch and establishing the correlation between its structure and functional properties have attracted extensive attentions.However,there are some leaving unanswered questions regarding the study of starch fine structure,such as accuracy determination of starch relative molecular weight,the ratio of amylose and amylopectin,accurate judgment of starch molecular chain conformation,aggregation state and branch composition.Native starch is difficult to be dissolved in cold water and most common organic solvents due to the abundant hydrogen bonds and dense crystalline structure of starch.Therefore,it is important to explore an effective solvent for starch to fully dissolve without structural damage for the accurate determination of starch structure.This paper aims to find a non-derivatized,low-consumption,high-efficiency,low-toxicity solvent for starch dissolution using ordinary corn starch as raw material,by exploring the dissolution mechanism,structural properties of starch in this solvent,and investigating the solubility difference of diverse starch in this solvent.The major study results are as follows:First,the solubility of ordinary corn starch in Li OH,Na OH and KOH solutions,alkali/thiourea and alkali/urea complex solvents were measured,and the chain conformation,static rheological behavior and dissolution mechanism of corn starch in complex solvent were explored.The present results show that KOH solution has the strongest dissolution ability to the starch compared to the same concentration of the Li OH and Na OH solutions,but the solubility of starch in a single alkali solution is still low(<50%)relative to in the dimethyl sulfoxide.However,the solubility of starch in 3 mol/L KOH/1 mol/L thiourea(3K1Thio)complex solvent reached 97.01%after 30 min at room temperature.Ordinary corn starch mainly exists in the form of monomolecular chain in the 3K1Thio complex solvent,which presents a star conformation.When starch is dissolved in 3K1Thio,OH~-in the solution can break down the inter-and intramolecular hydrogen bond network structure of starch,and thiourea surrounds the starch-KOH complex to prevent the occurrence of starch self-aggregation as a shell,thus increasing the fluidity of starch chain and promoting starch dissolution.Secondly,the solubility difference of starch with different amylose content in3K1Thio complex solvent was investigated,and the reasons for the diversity were analyzed.Our results show that the solubility of high amylose,ordinary and waxy corn starch in 3K1Thio complex solvent was 99.17%,97.01%and 89.68%,respectively,indicating that the solubility of starch in the complex solvent was positively correlated with amylose content.Waxy corn starch has higher branched chain content and crystallinity,stronger hydrogen bond interaction between starch molecules,and thus it is more difficult for solvent to destroy the hydrogen bond structure.In addition,the chain conformation of starch in the solvent is also related to the ratio of amylose and amylopectin,where high amylose corn starch tends to be in the flexible chain conformation,while waxy corn starch tends to be in the star conformation.Finally,the effects of 3K1Thio complex solvent on particle morphology,crystalline structure,group structure,thermal stability and chain length distribution of starch were analyzed.The granular and crystalline structures of the regenerated starch in 3K1Thio complex solvent are destroyed with significantly decreased crystallinity and the conversion from A-type pattern to V-type pattern.However,the solvent does not alter the molecular structure,chain length distribution and thermal stability of starch,and can be used for the determination of starch relative molecular weight and the characterization of structural parameters.The weight average molecular weight(Mw)was 2.88×10~7 g/mol when the ordinary corn starch was dissolved in 3K1Thio complex solvent. |
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